How to obtain a precise stroke in dynamic topology?
There are two settings that effect what you are trying to do.

The detail type method in dyntopo effects the way the detail size is calculated. Relative detail uses the onscreen pixel size as set in detail size when creating new edges and will give more detail the closer you zoom into your model. Changing to constant detail will use the same edge size as you zoom in and out. With constant detail the detail size is defined as "a percentage of blender units in object space", which means the objects scale property also plays a part in the size of the new edges.



The other option that will effect this is the brush radius. At the end of the radius slider there is a little padlock, enabling this will provide the same brush size relative to the model as you zoom in and out. The size is determined when you enable the padlock.